All calculations were performed using VASP. First-principles DFT calculations
were carried out using the PBEsol [1]} exchange-correlation
functional. The plane-wave cutoff was set to 600 eV. Phonon calculations were
performed using the Phonopy package [2]}. The primitive cells of
zirconia contain one, two, and four ZrO\(_2\)  formula units for the cubic,
tetragonal and monoclinic structures, respectively. A unit cell of 12 atoms
can therefore be used to describe all three phases, and a \(4\times 4\times 4\) 
Monkhorst-Pack \(k\) -mesh ensures energy convergence within less than 0.2 meV
per atom.
The structures were optimized until the forces were smaller than \(10^{-4}\)  eV
Å\(^{-1}\) .
